Measurement of the multi-jet cross-sections with the ATLAS detector at the LHC

(1)

UNIVERSITÀ DI PISA 1 34 3 IN S U PR EMÆ DIGN IT A T IS 1

GRADUATE COURSE IN PHYSICS

2

UNIVERSITY OF PISA

3

Graduate School of Basic Sciences

4 “GALILEO GALILEI” 5 XXIII ciclo 2007-2011 6

PhD

THESIS

7

Measurement of the multi-jet

8

cross-sections with the ATLAS

9

detector at the LHC

10

Candidate

Advisor

11

Zinonas Zinonos

Prof. Vincenzo Cavasinni

(2)

PhD Advisor :

_{Prof. Vincenzo Cavasinni}

_{- University of Pisa}

- & INFN Pisa

PhD Committee

President :

_{Prof. Kenichi Konishi}

_{University of Pisa}

Examinators :

_{Prof. Peter Loch}

_{- University of Arizona}

Prof. Elisabetta Barberio

- University of Melbourne

Prof. Franco Bedeschi

- INFN, Sezione di Pisa

Prof. Mauro Dell’Orso

- University of Pisa

Prof. Gavin Salam

- CERN

(3)

Dedicated to my wonderful wife

15

Louiza

(4)

Acknowledgements

I am most indebted to my advisor Vincenzo Cavasinni for his endless help,

144

precious guidance and continuous support. I am particularly grateful for his

145

extraordinary patience and kindness.

146

I thank the rest of the ATLAS Pisa group members, Nino Del Prete,

147

Chiara Roda, Calderini Giovanni, Paola Giannetti, Alberto Annovi, and my

148

colleagues Paolo Francavilla, Michelle Cascella, Federico Bertolucci, Francesco

149

Nuti, Francesco Crescioli and Daniele Puddu. I also owe thanks to the

ex-150

members of the Pisa team Iacopo Vivarelli, Andrea Dotti, Francesca Sarri,

Gi-151

angiobbe Vincent, Roberto Agostino Vitillo, Debenedetti Chiara and Claudia

152

Bertella.

153

I owe thanks to my friends outside of the Pisa team, Elisabetta Barberio,

Pe-154

ter Loch, Vassili Kazanine, Nikiforos Nikiforou, Natalie Heracleous,

Constanti-155

nos Melachrinos, Andreas Petrides, Monika Herodotou, Attikis Alexandros,

Je-156

had Mousa, Christos Hadjivasileiou, Maria Kameri, Maikantis Georgios, Giulio

157

Usai and Claudio Santoni. Their advice, encouragement, ideas and discussions

158

have been invaluable.

159

Special thanks goes to Nektarios Benekos for his exceptional help, inspiration

160

and fruitful discussions.

161

I thank my former advisor Prof. Ptochos Fotios, who first sparked my

inter-162

est in experimental high energy physics and planted my feet firmly on the path

163

to a career in physics.

164

Finally, I cannot forget my wonderful wife Louiza Manoli, whom I thank for

165

supporting me in pursuing a goal that I think cannot be reached without an

166

endless supply of love, understanding and encouragement.

167

(9)

(10)

Preface

The Large Hadron Collider (Lhc) is the world’s largest and highest-energy

par-169

ticle accelerator designed to accelerate and collide proton beams to the highest

170

energies and luminosities ever achieved in the history of experimental High

En-171

ergy Physics. The Lhc was built to help scientists to answer key unresolved

172

questions in particle physics and to test the known areas of physics at even

173

higher energy regimes.

174

For the past few decades, physicists have been able to describe with

increas-175

ing detail the fundamental particles that make up the nature at microscopic

176

scales and the fundamental interactions between them. This understanding is

177

encapsulated in the Standard Model of particle physics. Developed throughout

178

the mid to late 20th _{century, the current formulation was finalized in the mid}

179

1970s upon experimental confirmation of the existence of quarks. Since then,

180

discoveries of the bottom quark (1977), the Z and W gauge bosons (1983), the

181

top quark (1995), the tau neutrino (2000) and the great success in explaining a

182

wide variety of experimental results have given credence to the Standard Model.

183

Quantum chromodynamics (QCD), an important sector of the Standard

184

Model, is the theoretical framework built to formulate the strong interaction, a

185

fundamental force describing the interactions of the quarks and gluons,

collec-186

tively called partons, making up hadrons. A huge body of experimental evidence

187

for QCD has been gathered over the years in collider physics. Quarks and

glu-188

ons, being produced in large abundance at hadron colliders, like the Lhc, evolve

189

to form experimental signatures in detectors, the jets of the so-called hadronic

190

final state.

191

Jet production and properties are key observables in high-energy particle

192

physics. They have been measured so far in many beam colliders, such as

proton-193

proton, proton-antiproton, electron-positron and electron-proton both at GeV

194

(LEP, HERA) and TeV-energy scales (Tevatron) as well. They have provided

195

precise measurements of the strong coupling constant, have been used to obtain

196

information about the structure of the proton, and have become important tools

197

for understanding the strong interaction and searching for physics within and

198

beyond the Standard Model.

199

Measurements of the jet cross-section and characteristics in proton-proton

200

collisions at an unprecedented center-of-mass energy of 7 TeV at Lhc, were

201

recently performed by the Atlas experiment. The work constituted by this

202

thesis represents a part of these measurements; an endeavor to understand some

203

of the most important experimental aspects of the QCD theory, and in particular

204

the production of multi-jet events in proton-proton collisions.

205

This thesis is devoted to describing the analysis set up to measure the

multi-206

jet production rates and their kinematic features in early data provided by Lhc.

207

(11)

A comparison is made between the theoretical predictions and the experimental

208

results obtained by the dedicated analyses performed on the data collected by

209

the Atlas experiment. Special emphasis is laid on understanding the various

210

sources of systematic uncertainties on the experimental results, together with

211

the uncertainties of the theoretical calculations.

212

The author was actively involved in many efforts during the first acquisition

213

of √s = 900 GeV and then √s = 7 TeV proton-proton collision data, and in

214

several jet measurement analyses using the full 2010 dataset.

215

Starting in 2009, he studied the kinematic properties of jets produced in

216

pp collisions at √s = 900 GeV (presented at IFAE2010 and published by SIF

217

in Nuovo Cimento C [1]) and later he participated in the first observation of

218

energetic jets in√s = 7 TeV data collected in 2010 (presented at PLHC2010 [2]).

219

He also made a significant contribution to the first measurement of inclusive jet

220

production cross-section (presented at 35th _{ICHEP2010 [3], at HCP2010 [4]}

221

and published in EPJC [5]) and had a major role in the first measurement

222

of multi-jet production cross-sections (presented at HCP2010 [6]). Finally, in

223

2011, particular emphasis was placed on the measurement of the multi-jet

cross-224

sections with a larger data sample (presented at 46th _{Rencontres de Moriond}

225

on QCD and High Energy Interactions [7], at PisaJet2011 [8] and published in

226

EPJC [9]).

227

The author has participated in all phases of the multi-jet cross-section

anal-228

ysis, from the first measurement to the recent publications, and had many

re-229

sponsibilities during these efforts. Of course, this achievement was only possible

230

thanks to the outstanding competence, dedication and efforts of many people

231

working together in the multi-jet team.

232

Most of the produced results is the outcome of a collaboration, in which

233

the author had a leading role. In particular, he has been direct responsible for

234

the multi-jet trigger performance studies and the trigger design for the event

235

selection. He was responsible for calculating the total integrated luminosity

cor-236

responding to the data periods used in the analysis. He developed the Monte

237

Carlo machinery for generating all multi-jet observables and was responsible for

238

the massive production of simulated events with different leading-order Monte

239

Carlo programs attached to numerous tuning parameters. During the Monte

240

Carlo production, several studies were focused particularly on understanding the

241

differences observed between the simulated samples with different Monte Carlo

242

tunes. Emphasis was put on studying the effects resulting from the choice of the

243

parton distribution functions implemented in the simulation. Among his

respon-244

sibilities, was also the estimation of non-perturbative QCD correction factors

245

needed for the next-to-leading-order theoretical calculation using an enormous

246

amount of simulated events. A lot of contribution was given in understanding

247

the impact of implementing the non-perturbative corrections in the theoretical

248

predictions at next-to-leading-order and their effect on the overall theoretical

249

systematic errors. He participated in the special validation tasks undertaken to

250

control the theoretical calculations at leading-order and next-to-leading-order,

251

to test data unfolding factors, to extract the jet energy scale uncertainty in the

252

data, to understand the impact of pile-up on the measurements and to make

253

estimations of the overall systematic uncertainty bands.

254

He had also a major role in designing the cut-flow of the analysis, in

con-255

structing comparison tables and performing final cross-checks of all final results.

256

This thesis is organized as follows. Chapter 1 provides an introduction and

257

(12)

infrastructure at Cern, used to produce, collect, reconstruct and analyze the experimental data. Chapter 2 contains basic information about the Lhc, the

260

machine which accelerates and collides proton beams. Chapter 3 describes the

261

apparatus used to record and reconstruct the proton collision events: the Atlas

262

detector. A basic description is given on the detector’s functionality, both at

263

hardware and software level. Chapter 4 gives a general theoretical introduction

264

in the Standard Model and a brief review on the Higgs production mechanisms

265

in hadron colliders and decays. Also, this chapter provides an phenomenological

266

overview of Quantum Chromodynamics. The description of the main analysis

267

is done in Chapter 5. All small pieces of the analysis are coherently matched

to-268

gether in a big chapter, including relevant theoretical aspects and experimental

269

techniques, leading to the final results. An introduction in the multi-jet analysis

270

is given first in Section 5.1, followed by the definition of the cross-sections to be

271

measured (Section 5.2), a discussion of the simulations used in the measurement

272

and the theoretical predictions to which the data are compared (Section 5.3).

273

Event selection and reconstruction are described in Section 5.4, including

in-274

formation on the trigger performance, the jet reconstruction and pile-up. Data

275

correction is then described in Section 5.5. The evaluation of the main

uncer-276

tainties in the measurement, mainly coming from the jet energy scale, is given

277

is Section 5.6. The systematics in the theoretical predictions are described in

278

Section 5.7, followed by the results and conclusions in Sections 5.8 and 5.9.

279

In Section 5.8.8, the calculated multi-jet cross-sections are compared to other

280

Standard Model physics channels already measured or expected to be measured

281

by Atlas. The thesis is concluded in Chapter 6 by summarizing all findings

282

and making suggestions for future analyses.

283

(13)

Chapter 1

284

Introduction

285

At the Large Hadron Collider (Lhc), jet production is the dominant high

286

transverse-momentum process providing a direct test of Quantum

Chromody-287

namics (QCD) physics at the TeV scale. In fact, the Lhc physics program with

288

proton-proton (pp) collisions at 7 TeV center-of-mass energy, for the time

pe-289

riod 2010-2012, allows QCD physics to be tested in an entirely new high energy

290

regime.

291

One of the most striking features of Lhc final states is the large number

292

of events with several hard jets. QCD multi-jet events are being produced in

293

large abundance and multi-jet cross-sections are among the first measurements

294

possible to perform with real data collected by the Atlas detector. Lhc is

295

offering thus for the first time the possibility to explore perturbative QCD at

296

the largest center-of-mass energies and probe fundamental interactions at the

297

smallest distances ever achieved in the history of collider physics.

298

A detailed understanding of QCD jet production is very important for the

299

QCD gauge non-abelian theory itself and for all almost the physics processes

300

to be studied at the Lhc. The measurement of jet production cross-sections

301

at Lhc provides a stringent test of perturbative QCD in an unexplored energy

302

regime never probed so far. Extending the kinematic limit of the partonic hard

303

scale, Q2_{, at the TeV}2 _{order, leading and next-to-leading-order perturbative}

304

QCD predictions can be tested with experimental data. The jet cross-section

305

measurements can be thereby used as observables for the determination of the

306

running strong coupling constant up to TeV-scales with high accuracy, as well

307

as show sensitivity to the proton’s parton densities over a wide range of scale

308

and momentum fraction.

309

QCD studies are also relevant for the searches of physics of and beyond

Stan-310

dard Model; new physics. In fact, the multi-jet QCD events are a significant

311

background to many physics channels and being able to measure and describe

312

their features at this high energy regime is essential. The immense amount

313

of available phase-space and the large acceptance of the modern detectors like

314

Atlas and Cms, with calorimeters covering a region of almost 10 units of

pseu-315

dorapidity, can lead to the production and identification of final states with 10

316

or more jets. These kind of events would hide or strongly modify all possible

317

physics signals which involve multi-jet production in the final state, such as

top-318

antitop quark hadronic decays, di-tau lepton hadronic decays and vector boson

319

production in association with jets. Moreover, searches for new physics like

Su-320

(14)

persymmetry and new phenomena, typically require very large jet multiplicities

321

and hence they are fully exposed to the high production rate of QCD multi-jet

322

prompt processes. Consequently, a precise knowledge of final states which

in-323

volve significant hadronic activity is required. Precision measurements with pp

324

collision data and accurate predictions for QCD jet event rates (cross-sections

325

and distribution shapes) must be thus performed.

326

Jet cross-sections and properties are key observables in high-energy particle

327

physics. They have been measured at e+_e−_{, ep, p¯}_{p, and pp colliders, as well as}

328

in γp and γγ collisions. They have provided precise measurements of the strong

329

coupling constant, have been used to obtain information about the structure

330

of the proton and photon, and have helped to understand better the strong

331

interaction and search for physics beyond the Standard Model. Multi-jet

cross-332

sections measurements have been performed at the Tevatron in p¯p collisions at

333

1.96 TeV center-of-mass energy. Both experiments, Cdf [10, 11] and D0 [12, 13]

334

have measured the multi-jet cross-section, event shapes and invariant mass of

335

systems with up to 4 jets in the final state. Also, the Cms collaboration has

336

recently released measurements of the three-jet to two-jet cross-section ratios at

337

a 7 TeV center-of-mass energy [14].

338

In this thesis, the first measurements of inclusive multi-jet cross-sections

339

using the Atlas detector are presented. These measurements are performed

340

using a data set of pp collisions at 7 TeV center-of-mass energy, taken early in

341

Lhc running in 2010, corresponding to an integrated luminosity of ∼ 2.4 pb−1_.

342

The measurement involves a precise determination of the jet trigger and

recon-343

struction efficiencies of Atlas for jets, as well as a first determination of the

344

calorimeter response to jet energy, jet flavor and closest distance to other jets

345

in dense hadronic environments.

346

The Atlas detector is instrumented over almost the entire solid angle around

347

the pp collision point with layers of tracking detectors, electromagnetic and

348

hadronic calorimeters. The calorimeters are surrounded by the muon

spectrom-349

eter which consists of three large superconducting toroids, a system of precision

350

tracking chambers, and detectors for triggering. The inner detector, consisting

351

of silicon pixel and micro-strip detectors as well as a transition radiation tracker,

352

is immersed in a 2 T solenoidal magnetic field. Atlas has a three-level trigger

353

system, with the first level trigger being based on custom-built hardware and

354

the two higher level triggers being realized in software. Jet measurements are

355

made using a finely segmented hermetic calorimetric system, designed to provide

356

three-dimensional reconstruction of particle showers and detect high energy jets

357

with high efficiency and excellent energy resolution up to |η| . 4.9.

358

Individual jets are identified and built using the anti-ktjet algorithm with

359

two jet resolution parameters, R = 0.4 and 0.6. This algorithm is well-motivated

360

since it can be implemented in next-to-leading-order perturbative QCD

calcu-361

lations, is collinear and infrared-safe to all orders and produces geometrically

362

well-defined “cone-like” jets. The inputs to this algorithm are three-dimensional

363

clusters of calorimeter cells with energy depositions significantly above the

mea-364

sured noise. Jet four-momenta are constructed as the vectorial sum of clusters

365

of cells, treating each cluster as a four-vector with zero mass, assuming that the

366

corresponding particle stems from the primary vertex. The jet four-momenta

367

are then corrected as a function of pseudorapidity and transverse energy for

368

various effects, the largest of which are the hadronic shower response and the

369

detector material distribution. This is done using an energy calibration scheme

(15)

3 based on Monte Carlo studies including full detector simulation and validated

371

with extensive test-beam and collision data studies.

372

The event selection in the analysis starts with the first-level two and

three-373

jet triggers which collect events that have at least two or three large

trans-374

verse energy depositions in the calorimeters. In the dataset used, the two-jet

375

triggers were prescaled. So, in order to achieve the highest possible effective

376

integrated luminosity, the inclusive two-jet events have been measured by

us-377

ing an exclusive combination of all di-jet triggers available in that data period.

378

The candidate multi-jet events are then required to satisfy certain data quality

379

criteria, to eliminate various detector effects, to suppress beam and other

non-380

collision backgrounds, and have a primary collision vertex defined by multiple

381

charged-particle tracks.

382

Cross-sections are calculated in bins of inclusive jet multiplicity, meaning

383

that an event is counted in a jet multiplicity bin if it contains a number of jets

384

that is equal to or greater than that multiplicity. Inclusive multiplicity bins

385

are basically used because they are stable in the perturbative QCD fixed-order

386

calculation, unlike exclusive bins. Only jets in the kinematic region pT > 60 GeV

387

and |y| < 2.8 are counted in the measurement. These cuts are chosen to ensure

388

that the jets are reconstructed with high efficiency and they are in a kinematic

389

region where the jet energy scale is well understood. The leading jet is further

390

required to pass a higher transverse momentum cut, at 80 GeV, so as to stabilize

391

the next-to-leading-order perturbative QCD calculations in the dijet case.

392

All measured multi-jet cross-sections are corrected for all experimental effects

393

using an unfolding technique with simulated events, to allow comparisons with

394

particle-level theoretical predictions.

395

The theoretical motivation for measuring multi-jet final states is twofold; to

396

evaluate the robustness of the leading-order perturbative QCD calculations in

397

representing the high jet multiplicity events and to test fixed-order perturbative

398

QCD calculations at next-to-leading-order. For the leading-order comparisons,

399

events with up to six jets in the final state are studied. For the

next-to-leading-400

order perturbative QCD study, the focus is on three-jet events and their

com-401

parison to two-jet events.

402

Many different effects are included in leading-order Monte Carlo simulations

403

of jet production at the Lhc. These include the modeling of the underlying

404

event and hadronization which can affect the cross-section calculation through

405

their impact on the jet kinematics. Effects arising from differences between

406

the matrix-element (with up to 2 → 6 matrix-element scattering diagrams)

407

plus parton-shower calculation and the parton-shower calculation alone (with

408

only 2 → 2 matrix-element scattering diagrams) also need to be understood.

409

These topics are not easily separable, since tuning of some of the effects, such

410

as the underlying event, to data is needed. The tuning process, however,

auto-411

matically fixes other inputs in the Monte Carlo simulation, such as the parton

412

distribution functions, the parton-shower model and the hadronization model.

413

The non-triviality to perfectly separate out some effects introduces a difficulty

414

in obtaining a full estimate of the theoretical uncertainty associated with the

415

leading-order Monte Carlo predictions. Therefore, one of the aims of this

anal-416

ysis is to test the performance of the different leading-order Monte Carlo

sim-417

ulations, so that they can be used to estimate multi-jet backgrounds for new

418

particle searches, not to discern whether deviations with respect to QCD are

419

present in the data. The latter goal is best achieved by comparing with

(16)

to-leading-order perturbative QCD calculations.

421

The next-to-leading perturbative QCD calculation used in this analysis, is

422

not interfaced to a Monte Carlo simulation with hadronization and other

non-423

perturbative effects and hence making practically the predicted partonic

cross-424

sections unmeasurable. For comparison with data at the particle-level

simula-425

tion, soft (non-perturbative) corrections must be applied to the next-to-leading

426

perturbative QCD calculation. This is done using leading-logarithmic parton

427

shower Monte Carlo programs in which the soft QCD effects are enabled and

428

disabled, and then evaluating the relative ratio.

429

For high multiplicity studies, which include events with up to six jets, the

430

resolution parameter in the jet reconstruction is fixed to R = 0.4 to contend with

431

the limited phase-space and to reduce the impact of the underlying event in the

432

jet energy determination. For testing the next-to-leading-order perturbative

433

QCD calculations, where the study focuses on three-jet events, a resolution

434

parameter of R = 0.6 is preferred, since a larger value of R is found to be less

435

sensitive to theoretical scale uncertainties.

436

The cross-section as a function of the inclusive jet multiplicity of up to six

437

jets is measured. A study that substantially reduces the impact of systematic

438

uncertainties is the ratio of the N -jet to (N − 1)-jet cross-section as a function

439

of multiplicity. In this ratio, the impact of the jet energy scale uncertainty is

440

significantly reduced.

441

Multi-jet kinematic features are also explored. Differential cross-sections for

442

multi-jet production are measured as a function of transverse momentum of the

443

leading, second leading, third leading and fourth leading jet, and their scalar

444

sum as well.

445

A measurement with particular sensitivity to limitations in the leading-order

446

Monte Carlo simulations and next-to-leading-order perturbative QCD

calcula-447

tions is the ratio of the inclusive three-to-two-jet differential cross-section as a

448

function of different kinematic variables. In this measurement, uncertainties in

449

the jet energy scale are significantly suppressed. The three-to-two-jet ratio as a

450

function of the leading jet transverse momentum can be used to tune the Monte

451

Carlo simulations for effects due to final state radiation. The three-to-two-jet

452

ratio as a function of the transverse momentum scalar sum of two leading jets,

453

is found to give the smallest theoretical scale uncertainty and is, therefore, most

454

sensitive to input parameters such as the strong coupling constant αS.

455

A dedicated study of angular distributions in inclusive three-jet systems is

456

also performed. Polar and azimuthal differences between the first three leading

457

jets are measured in data and compared to simulation using different leading

458

and next-to-leading-order Monte Carlo programs.

459

Special emphasis is given on the evaluation of the jet energy scale and its

460

uncertainty, which is the dominant uncertainty source for most results presented

461

in this analysis. The fact that cross-sections are steeply falling as a function

462

of jet transverse-momentum implies that, even a relatively small uncertainty in

463

the determination of the jet transverse-momentum translates into a substantial

464

change in the cross-sections as events may migrate along the steeply falling

465

curve.

466

The jet energy scale and its uncertainty have been determined for jets from a

467

dijet sample without nearby activity in the calorimeter. For a multi-jet analysis,

468

additional systematic uncertainties need to be considered. These uncertainties

469

arise from the difference in the calorimeter response to jets of different flavors

(17)

5 as well as the impact of the presence of nearby activity in the calorimeter on

471

the jet energy measurement.

472

For events containing two or more jets, a reasonable agreement is found

473

between data and leading-order Monte Carlo simulations with parton-shower

474

tunes that describe adequately the Atlas√s = 7 TeV underlying event data.

475

The agreement is found after the predictions of the Monte Carlo simulations are

476

normalized to the measured inclusive two-jet cross-section.

477

All models reproduce the main features of the multi-jet data. The 2 → 2

478

QCD calculations show some departure from the data for the three-to-two jet

479

cross-section ratios, predicting a higher ratio than observed. The 2 → n ≤ 6

480

matrix-element calculations sufficiently describe the measured ratios,

indepen-481

dently of the Monte Carlo tune or parton-shower implementation.

482

The shape of the differential cross-sections as a function of transverse

mo-483

mentum and the scalar sum of transverse momenta, studied in the inclusive

484

two-jet and three-jet bins, drops off less (more) steeply in the 2 → n (2 → 2)

485

calculations. A measurement of the three-to-two-jet cross-section ratio as a

486

function of the leading jet transverse momentum and the sum of the two

lead-487

ing jet transverse momenta is better described by multi-leg parton programs.

488

Three-to-two differential cross-section ratios are compared to perturbative

next-489

to-leading-order QCD theoretical predictions, showing a reasonable agreement

490

with data.

491

Future comparisons with next-to-leading-order QCD calculations will

pro-492

vide useful information for constraining parameters, such as parton distribution

493

functions or the value of the strong coupling constant, αS. Systematic

un-494

certainties from the measurement are presently comparable to the theoretical

495

uncertainties, but should be reduced with larger data samples and better control

496

of all sources of systematic uncertainty.

(18)

(19)

Chapter 2

498

The Large Hadron Collider

499

2.1 A Short Introduction to Lhc

500

The Large Hadron Collider (Lhc) at Cern near Geneva is the world’s newest

501

and most powerful tool for Particle Physics research [15, 16, 17]. It is designed

502

to collide proton beams with a center-of-mass energy of√s = 14 TeV and an

503

unprecedented luminosity of 1034_cm−2_s−1_{, a factor of ∼ 7 in energy and 100 in}

504

luminosity larger than Tevatron at Fermilab. It can also collide heavy (P b) ions

505

with an energy of 2.8 TeV per nucleon and a peak luminosity of 1027_cm−2_s−1_.

506

The colliding beam can contain up to 2808 proton bunches with up to 1.1 ×

507

1011_{protons each. The bunch-crossing rate will be 40 MHz at an instantaneous}

508

luminosity of Linst = 2 × 1033cm−2s−1 in the low luminosity phase and Linst =

509

1034_cm−2_s−1 _{in the high luminosity phase.}

510

The limiting factor to the achievable center-of-mass energy is the bending

511

power needed to keep the proton beams circulating in the 27 km-circumference

512

of the LEP tunnel. From the equation

513

p(TeV) ≃ 0.3B(T)R(km) (2.1)

where p is the beam momentum, B the magnetic filed provided by the magnets

514

of the accelerating machine and R ≃ 4.3 km is the radius of the Lhc ring, it is

515

deduced that the required magnetic field strength to achieve a beam momentum

516

of 7 TeV is about 5.4 T. In practice, since the machine cannot be completely

517

filled with magnets, the needed bending power is obtained with about 1200

su-518

perconducting dipoles providing a maximum magnetic field of 8.4 T at cryogenic

519

temperatures, which represents a very ambitious technological challenge.

520

The total inelastic proton-proton (pp) cross-section is approximately 80 mb

521

at √s = 14 TeV, meaning that Lhc will produce collision events at very high

522

rates. In particular, the event rate R, defined as the number of events produced

523

per second by the pp interactions, is expected to be

524

R = σ × L ≃ 80 mb × 1034 cm−2_s−1_{≃ 10}9_events/s _(2.2)

when running at high luminosity.

525

On average 5 pp (low luminosity phase) and 25 pp (high luminosity phase)

526

interactions are expected per bunch collision. The most important nominal

527

parameters for the collider are summarized in Table 2.1.

528

(20)

Quantity Value

Circumference 26659 m

Dipole operating temperature 1.9 K

Number of magnets 9593

Number of main dipoles 1232

Number of main quadrupoles 392

Number of RF cavities 8 per beam

Nominal energy, protons 7 TeV

Nominal energy, ions 2.76 TeV per nucleon

Peak magnetic dipole field 8.33 T

Min. distance between bunches 7 m

Design luminosity 1034_cm−2_s−1

No. of bunches per proton beam 2808

No. of protons per bunch 1.15 × 1011

Number of turns per second 11245

Number of collisions per second 600 × 106

Stored energy per beam during collisions 362 MJ

Events per bunch crossing 19

Inelastic cross-section 60.0 mb

Total cross-section 80 − 100.0 mb

Table 2.1: Nominal parameters of the Lhc.

(21)

2.1. A SHORT INTRODUCTION TO LHC 9 Figure 2.1 shows an overview of the accelerator complex at Cern.

529

The protons are accelerated in several steps in the already existing

accelera-530

tor facilities. The protons originate from a hydrogen source and are accelerated

531

in the Linear Accelerator (LINAC) to an energy of 5 MeV. In the synchrotron

532

booster the protons obtain an energy of 1.4 GeV, then they are transferred into

533

the Proton Synchrotron (PS) and accelerated to 25 GeV further and finally

534

the energy is increased to 450 GeV in the Super Proton Synchrotron (SPS).

535

From the SPS they are injected into the Lhc ring where they are eventually

536

accelerated in RF-cavities up to their final energy.

537

For the acceleration there are eight RF-cavities installed per beam. Each

538

cavity provides an acceleration voltage of 2 MV at an operation frequency of

539

400 MHz. The bunch spacing in time is 25 ns. The cavities operate at a

540

temperature of 4.5 K. The two beams are accelerated in opposite directions

541

and have separate magnetic channels in the superconducting dipole magnets.

542

This is achieved by two apertures and an eight-shaped magnetic field. Both

543

beams share the same yoke and cryostat system.

544

In total there are 9593 installed superconducting magnets of which 1232 are

545

dipole magnets, 500 are quadrupole and 4000 are corrector magnets. The dipole

546

magnets are made of copper-clad niobium-titanium cables. With a total current

547

in the dipoles of 11.7 kA, a peak field of 8.33 T can be reached. The magnets

548

are cooled to a temperature of 1.9 K with super-fluid helium in a vacuum-vessel

549

contained cryostat. The beams travel in a beam pipe, which is held at a pressure

550

of 10−13 _{atm. In the collision areas the beam pipe is made of beryllium. The}

551

beams are brought to collision at four points along the ring at which detectors

552

are positioned. Atlas and Cms are two multiple-purpose detectors that cover

553

a broad range of physics, whereas the Alice detector aims at heavy-ion physics

554

and LHCb at B-physics [17].

555

The Lhc construction was completed in 2008 and started the same year with

556

first beams. Due to problems with the dipole magnets at currents needed for

557

7 TeV-beams, the initial center-of-mass energy was planned to be√s = 10 TeV.

558

However, an incident in September 2008 with a superconductive connection bar

559

between two dipole magnets stopped the further running of the Lhc. This

560

connection bar inside the helium vessel had a small resistance that lead to

561

an electrical arc and it destroyed the helium enclosure. The sudden escape and

562

expansion of helium into the tunnel to atmospheric pressure lead to a mechanical

563

shock wave, which dislocated and damaged 58 of the dipole magnets. This

564

incident lead to a further decrease of the center-of-mass energy to a safer level

565

of √s = 7 TeV. After several months of repair, in November 2009 first pp

566

collisions were achieved with collision energies of up to√s = 2.36 TeV. In early

567

2010, the machine restarted and operating normally with beams and collisions

568

for a physics run with a collision energy of√s = 7 TeV.

569

By November 2011, Lhc has achieved the following operational records

570

Peak Stable Luminosity Delivered: 3.65 × 10

33_cm₋₂_s₋₁

571

Maximum Luminosity Delivered to ATLAS in one fill : 122.44 pb−1

572

Maximum Colliding Bunches: 1854

573

Maximum Peak Events per Bunch Crossing: 33.96

574

Maximum Average Events per Bunch Crossing: 32.21

(22)

By the end 2012 the √s = 7 TeV phase will be completed and a one year

576

shutdown with possible upgrades of the detectors will follow. In 2012 it is

577

foreseen to reach the design beam energies and design luminosity.

578

2.2 Impediments to High Luminosity

579

Hadron colliders employ bunched beams [18]. If two bunches containing n1and

580

n2 particles collide head-on with frequency f , instantaneous luminosity L is

581 given as 582 L = f_4πσn1n2 xσy (2.3)

where σx and σy characterize the transverse beam profiles in the horizontal

583

(bend) and vertical directions. In this form, it is assumed that the bunches are

584

identical in transverse profile, that the profiles are independent of position along

585

the bunch and the particle distributions are not altered during bunch passage.

586

The single particle transverse motion in a alternating-gradient synchrotron

587

like Lhc, is not a simple sinusoid and rather it may be expressed in the form

588

x(s) = Apβ(s) cos[ψ(s) + δ] (2.4)

where s is path length in the beam direction, A and δ are constants of integration

589

and the envelope of the motion is modulated by the amplitude function, β. The

590

phase advances according to dψ/ds = 1/β; that is, β also plays the role of a

591

local wavelength λ/2π, and the tune ν is the number of such oscillations per

592

turn about the closed path. In the neighborhood of an interaction point, the

593

beam optics of the ring is configured so as to produce a near focus. The value

594

of the amplitude function at this point is designated β∗_.

595

The motion as it develops with s describes an ellipse in {x, dx/ds}

phase-596

space, the area of which is πA2_{. If the interior of that ellipse is populated by}

597

an ensemble of particles, given the parameter “emittance” and denoted by ǫ,

598

the area would change only with beam energy in the absence of other processes.

599

For a beam with a Gaussian distribution in {x, dx/ds}, the area containing one

600

standard deviation σx is the definition of emittance

601 ǫx= π σ2 x βx (2.5) with a corresponding expression in the other transverse direction, y. This

defi-602

nition includes ∼ 40% of the beam.

603

To complete the coordinates used to describe the motion, the longitudinal

604

variables {z, δp/p} is added in the transverse phase-space {x, dx/ds, y, dy/ds},

605

where z is the distance by which the particle leads the “ideal” particle along

606

the design trajectory. Radiofrequency electric fields in the s direction provide

607

the means for longitudinal oscillations, and the frequency determines the bunch

608

length. The quantity δp/p is characterized as “energy spread”.

609

In hadron collisions, the “bunch length” is a significant quantity for a

va-610

riety of reasons. If the bunch length becomes larger than β∗ _{the luminosity is}

611

adversely affected. This is because β grows parabolically as one proceeds away

612

from the interaction point and so the beam size increases thus lowering the

613

contribution to the luminosity from such locations.

(23)

2.2. IMPEDIMENTS TO HIGH LUMINOSITY 11 Equation 2.3 can be recast in terms of emittance and amplitude functions

615 as 616 L = f₄p n1n2 ǫxβx∗ǫyβy∗ (2.6) Therefore, to achieve high luminosity, all one has to do is make high

popula-617

tion bunches of low emittance to collide at high frequency at locations where

618

the beam optics provides as low values of the amplitude functions as possible.

619

While there are no fundamental limits to this process, there are certainly several

620

technical challenges to meet, mostly related to the structure of the acceleration

621

machine.

(24)

(25)

Chapter 3

623

Overview of the ATLAS

624

Detector

625

The Atlas detector is one of the two general purpose detectors constructed for

626

the Large Hadron Collider at Cern in Geneva. The detector is designed to be

627

sensitive to the full range of high pT physics processes occurring in√s = 14 TeV

628

proton-proton collisions and at high luminosity of 1034 _cm₋₂_s₋₁_.

629

The hermetic Atlas detector, depicted by Figure 3.1, is composed of a

cen-630

tral tracker, a calorimeter system (electromagnetic and hadronic) and of a large

631

muon spectrometer. A detailed description of the detector is well documented

632

at Refs.[19, 20, 21, 22].

Figure 3.1: Cut-away view of the Atlas detector. The dimensions of the detector

are ∼ 25 m in height and ∼ 44 m in length. The overall weight of the detector is approximately 7000 tonnes.

633

The physics program of Atlas can be principally sorted into four categories.

634

The first covers the SM and flavor physics, Higgs explorations, searches for

635

(26)

η-coverage

Detector Component Required Resolution Measurement L1 Trigger

Tracking σpT pT = 0.05%pT ⊕ 1% ±2.5 -EM calorimetry σE E = 10% √ E ⊕ 0.7% ±3.2 ±2.5 Hadronic calorimetry

Barrel & End-Cap σE

E = 50% √ E ⊕ 3% ±3.2 Forward σE E = 100% √ E ⊕ 10% 3.1 < |η| < 4.9 Muon spectrometer σpT pT = 10% ±2.7 ±2.4

Table 3.1: Atlas design performance requirements [22]. The Muon spectrometer

performance is quoted for a muon with pT = 1 TeV, measured in stand-alone mode,

independently of the Inner Detector.

physics beyond the SM and new phenomena like exotic physics and gravitation

636

at tera-scales. In the second category belongs the physics of heavy ion collisions.

637

To accommodate this rich physics program, Atlas has a general-purpose design,

638

capable of detecting and measuring different types of particles. The main design

639

aims of Atlas can be summarized as follows [19, 20]:

640

Fast, radiation-hard electronics and sensors with high granularity;

641

Excellent momentum resolution, detector efficiency and vertex

identifica-642

tion;

643

Electromagnetic calorimetry for electron and photon measurements, and

644

full coverage hadronic calorimetry for jet and missing ET measurements;

645

Muon identification and measurement over a wide range of energies;

646

High efficiency triggers with excellent background rejection, capable of

647

working with low kinematic thresholds in a high multiplicity environment;

648

High energy/momentum resolution in all sub-detectors, summarized in

649

Table 3.1.

650

All of this must be achieved with high precision in the challenging

environ-651

ment set by the Lhc machine, with up to an anticipated average number of 23

652

inelastic pp collisions per 25 ns bunch crossing, consisting mostly of inelastic

653

QCD processes.

654

Emphasis is given on efficient tracking identification of charged particles and

655

accurate, large acceptance calorimetric measurement of shower pT and EmissT .

656

3.1 Kinematic Definitions

657

Throughout the following descriptions of the Atlas detector, cylindrical

co-658

ordinates R and φ are mostly used in the transverse plane x − y. In Atlas

659

detector, the positive x-axis is defined as pointing from the interaction point to

660

the center of the LHC ring, the positive y-axis is defined as pointing vertically

661

upwards, and the positive z-axis corresponds to protons running anti-clockwise.

(27)

3.1. KINEMATIC DEFINITIONS 15 The polar angle θ is measured from the beam axis (z-axis), the azimuthal angle

663

φ is measured in the transverse x − y-plane.

664

In experimental particle physics, a convenient set of variables to describe the

665

kinematics of measured objects is the Lorentz-invariant pseudorapidity η and

666

transverse component of momentum pT and the azimuthal angle φ.

667

Instead of polar angle θ, pseudorapidity is a commonly used spatial

coordi-668

nate describing the angular direction of a particle relative to the beam axis. It

669 is defined as 670 η = − ln tanθ 2 (3.1) where θ is the angle between the particle momentum vector p and the beam

671

axis. In terms of the momentum p = (px, py, pz), the pseudorapidity variable

672 can be written as 673 η = 1 2ln |p| + pL |p| − pL (3.2)

where pL= pZ is the component of the momentum along the beam axis.

There-674

fore, pseudorapidity depends only on the polar angle of the object’s trajectory,

675

and not on its energy. It is straightforward to express η as a function of rapidity

676

y and vice versa. From the definition of η, we have

677 eη= s |p| + pz |p| − pz (3.3) and 678 e−η₌ s |p| − pz |p| + pz (3.4) which combined together give

679

|p| = pTcosh η (3.5)

where pT is the magnitude of the transverse momentum

680

pT =

p

p2_{− p}2

z. (3.6)

Subtracting Equation 3.4 from Equation 3.3, we obtain

681

pz= pTsinh η. (3.7)

Using these results, one can define the rapidity variable y in terms of the

pseu-682

dorapidity variable η as follows

683 y = 1 2ln q p2 Tcosh 2_{η + m}2_{+ p} Tsinh η q p2 Tcosh 2_{η + m}2_{− p} Tsinh η (3.8)

where m is the rest mass of the measured particle or object. Conversely, the

684

pseudorapidity η can be expressed in terms of the rapidity y by

685 η =1 2ln q p2 Tcosh 2 y − m2_{+ m} Tsinh y q p2 Tcosh 2 y − m2_{− m} Tsinh y (3.9)

(28)

where 686 mT = E cosh y = pz sinh y (3.10)

is conventionally called the “transverse mass” of particle with energy E, given

687

by

688

m2T = m2+ p2x+ p2y. (3.11)

So, rapidity y can be also defined as

689 y = 1 2ln E + pL E − pL = tanh−1 pL E (3.12) In the high relativistic limit, p ≫ m, the rapidity defined in Equation 3.8

690

may be expanded to approximately obtain

691 y ≃ − ln tanθ 2 ≡ η (3.13)

getting thus back the expression of Equation 3.1. The pseudorapidity η is

ap-692

proximately equal to the rapidity y for relativistic objects, p ≫ m and θ ≫ 1/γ,

693

and in any case can be measured in detectors when the mass and momentum of

694

the particle are unknown. From this definition, one can also obtain the identities

695

sinh η = cot θ, cosh η = 1/ sin θ, tanh η = cos θ. (3.14)

Often, one speaks of the “forward” direction in a hadron collider experiment,

696

which refers to regions of the detector that are close to the beam axis, at high

697

η. The difference in the pseudorapidity of two objects is independent of Lorentz

698

boosts along the longitudinal axis.

699

Te radial distances between two points in the η − φ plane are often denoted

700

by

701

R2= (δη)2+ (δφ)2. (3.15)

In some cases, it is more convenient to define transverse energy as the energy

702

deposited in a calorimeter component, corrected for its position by the formula

703

ET = E/ cosh η. (3.16)

In the highly relativistic limit, E ≫ m, and neglecting calorimeter resolution

704

effects, the deposited ET is equal to the pT of the incident particle.

705

3.2 ATLAS Magnets and Magnetic Field

706

Atlas features a unique hybrid system of four large superconducting magnets.

707

This magnetic system is 22 m in diameter and 26 m in length, with a stored

708

energy of 1.6 GJ. After approximately 15 years of design, construction in

indus-709

try, and system integration at Cern, the system was installed and operational

710

in the underground cavern in 2007.

711

Figure A.1 shows the general layout, the four main layers of detectors and the

712

four superconducting magnets which provide the magnetic field over a volume of

713

approximately 12000 m3_{(defined as the region in which the field exceeds 50 mT).}

714

The Atlas magnet system, whose main parameters are listed in Table A.1,

715

consists of:

(29)

3.2. ATLAS MAGNETS AND MAGNETIC FIELD 17

a solenoid (Section 3.2.1), which is aligned on the beam axis and provides

717

a 2 T axial magnetic field for the inner detector, while minimizing the

718

radiative thickness in front of the barrel electromagnetic calorimeter;

719

a barrel toroid (Section 3.2.2) and two end-cap toroids (section 2.1.3),

720

which produce a toroidal magnetic field of approximately 0.5 T and 1 T

721

for the muon detectors in the central and end-cap regions, respectively.

722

3.2.1 Central Solenoid

723

The central solenoid [23, 24] is designed to provide a 2 T axial field (1.998 T at

724

the magnet’s center at the nominal operational current of 7.730 kA. To achieve

725

the desired calorimeter performance, the layout was carefully optimized to keep

726

the material thickness in front of the calorimeter as low as possible, resulting in

727

the solenoid assembly contributing a total of ∼ 0.66 radiation lengths at normal

728

incidence. This requires, in particular, that the solenoid windings and LAr

729

calorimeter share a common vacuum vessel, thereby eliminating two vacuum

730

walls. An additional heat shield consisting of 2 mm thick aluminium panels is

731

installed between the solenoid and the inner wall of the cryostat. The

single-732

layer coil is wound with a high-strength aluminium-stabilized N bT i conductor,

733

specially developed to achieve a high field while optimizing thickness, inside a

734

12 mm thick aluminium support cylinder.

735

The inner and outer diameters of the solenoid are 2.46 m and 2.56 m and

736

its axial length is 5.8 m. The coil mass is 5.4 tonnes and the stored energy is

737

40 MJ. The stored-energy-to-mass ratio of only 7.4 kJ/kg at nominal field clearly

738

demonstrates successful compliance with the design requirement of an extremely

739

light-weight structure. The flux is returned by the steel of the Atlas hadronic

740

calorimeter and its girder structure. The solenoid is charged and discharged in

741

about 30 minutes. In the case of a quench, the stored energy is absorbed by

742

the enthalpy of the cold mass which raises the cold mass temperature to a safe

743

value of 120 K maximum. Re-cooling to 4.5 K is achieved within lees than one

744

day.

745

The electromagnetic forces are counteracted by the combination of the coil

746

and warm-to-cold mechanical support, which maintains the concentricity of the

747

windings. All solenoid services pass through an S-shaped chimney at the top of

748

the cryostat, routing the service lines to the corresponding control dewar.

749

3.2.2 Toroid

750

The Atlas Toroid Magnet system consists of eight Barrel coils housed in

sepa-751

rate cryostats and two end-cap cryostats housing eight coils each. The end-cap

752

coils systems are rotated by 22.5◦ _{with respect to the Barrel Toroids in order}

753

to provide radial overlap and to optimize the bending power in the interface

754

regions of both coil systems.

755

Barrel Toroid

756

The main parameters of the magnet are listed in Table A.1. The cylindrical

757

volume surrounding the calorimeters and both end-cap toroids (see Figure A.1)

758

is filled by the magnetic field of the barrel toroid, which consists of eight coils

(30)

assembled radially and symmetrically around the beam axis and encased in

in-760

dividual racetrack-shaped, stainless-steel vacuum vessels (see Figure A.2). The

761

coil assembly is supported by eight inner and eight outer rings of struts. The

762

coils are of a flat racetrack type with two double-pancake windings made of

763

20.5 kA Al-stabilized N bT i superconductor. Each coil has an axial length of

764

25.3 m and extends radially from 9.4 m to 20.1 m. The total assembly weights

765

about 830 tonnes. The peak field provided by the Barrel Toroid coils is 3.9 T,

766

providing 2 to 6 Tm of bending power in the pseudorapidity range from 0 to

767

1.3.

768

The conductor and coil-winding technology is essentially the same in the

769

barrel and end-cap toroids; it is based on winding a pure Al-stabilized N bT i/Cu

770

conductor into pancake-shaped coils, followed by vacuum impregnation.

771

The cool down of the 360-tonne cold mass to 4.6 K takes about five weeks.

772

The net Lorentz forces of approximately 1400 tonnes per coil directed inwards

773

and the self-weight of the toroids are counteracted by the warm structure of

774

Al-alloy struts mounted in between the eight coils.

775

End-cap Toroids

776

The Atlas end-cap Toroid systems (Figure A.3) consists of eight coils

assem-777

bled radially and symmetrically around the beam axis. The coils are of a flat

778

racetrack type with two double-pancake windings made of 20.5 kA Al-stabilized

779

N bT i superconductor. They are cold-linked and assembled as a single cold mass

780

in one large cryostat. The cryostat rests on a rail system facilitating the

move-781

ment and parking for access to the detector center. Each coil has an axial length

782

of 5 m and extends radially from 1.65 m to 10.7 m. The total assembly weights

783

about 239 tonnes. The peak field provided by the Barrel Toroid coils is 4.1 T,

784

providing 4 to 8 Tm of bending power in the pseudorapidity range from 1.6 to

785

2.7.

786

3.3 Tracking System

787

3.3.1 Overview of the ATLAS Inner Detector

788

The Atlas Inner Detector (ID) [25] is designed to provide hermetic and

ro-789

bust pattern recognition, excellent momentum resolution and both primary and

790

secondary vertex measurements for charged tracks above a given pT threshold

791

(nominally 0.5 GeV within the pseudorapidity range |η| < 2.5 and full azimuthal

792

coverage). It also provides electron identification over |η| < 2.0 and a wide range

793

of energies (between 0.5 GeV and 150 GeV). This performance is required at

794

very dense environments and at highest luminosities expected from pp collisions

795

at Lhc. The general ID layout, as shown in Figure 3.2, reflects the performance

796

requirements.

797

The detector has been designed to provide a transverse momentum

resolu-798

tion, in the plane perpendicular to the beam axis, of σpT/pT = 0.05%pT GeV ⊗

799

1% and a transverse impact parameter resolution of 10µm for high momentum

800

particles in the central η region.

801

The Atlas Inner Detector combines high-resolution detectors at the inner

802

radii with continuous tracking elements at the outer radii, all contained in the

(31)

3.3. TRACKING SYSTEM 19

Figure 3.2: A 3D model of the Atlas Inner Detector.

Central Solenoid, which provides a nominal field of 2 T. The highest

granu-804

larity is achieved around the vertex region using semiconductor pixel detectors

805

followed by a silicon micro-strip detector. Typically for each track the pixel

806

detector contributes three and the strips four space points. At larger radii

typ-807

ically 36 tracking points are provided by the straw tube tracker. The relative

808

precision of the measurement is well matched, so that no single measurement

809

dominates the momentum resolution. The outer radius of the Inner Detector

810

is 1.15 m, and the total length 7 m. In the barrel region the high-precision

811

detectors are arranged in concentric cylinders around the beam axis, while the

812

end-cap detectors are mounted on disks perpendicular to the beam axis. The

813

barrel TRT straws are parallel to the beam direction. All end-cap tracking

814

elements are located in planes perpendicular to the beam direction.

815

The Inner Detector comprises three complementary sub-detectors: the Pixel

816

Detector, the Semiconductor Tracker and the Transition Radiation Tracker.

817

Relevant features are described briefly below.

818

3.3.2 Pixel Detector

819

It consists of sensitive elements cover radial distances between 50.5 mm and 150

820

mm. The detector consists of 1744 silicon pixel modules [22] arranged in three

821

concentric barrel layers and two end-caps of three disks each. It provides

typi-822

cally three measurement points for particles originating in the beam-interaction

823

region. Each module covers an active area of 16.4 mm × 60.8 mm and contains

824

47232 pixels, most of size 50µm×400µm. The direction of the shorter pitch

de-825

fines the local x-coordinate on the module and corresponds to the high-precision

826

position measurement in the r − φ plane. The longer pitch, corresponding to

827

the local y-coordinate, is oriented approximately along the z direction in the

828

barrel and along r in the end-caps. A module is read out by 16 radiation-hard

Measurement of the multi-jet cross-sections with the ATLAS detector at the LHC

GRADUATE COURSE IN PHYSICS

UNIVERSITY OF PISA

PhD

THESIS

Measurement of the multi-jet

cross-sections with the ATLAS

detector at the LHC

Candidate

Advisor

Zinonas Zinonos

Prof. Vincenzo Cavasinni

PhD Advisor :

Prof. Vincenzo Cavasinni

- University of Pisa

- & INFN Pisa

PhD Committee

President :

Prof. Kenichi Konishi

University of Pisa

Examinators :

Prof. Peter Loch

- University of Arizona

Prof. Elisabetta Barberio

- University of Melbourne

Prof. Franco Bedeschi

- INFN, Sezione di Pisa

Prof. Mauro Dell’Orso

- University of Pisa

Prof. Gavin Salam

- CERN

Contents

Acknowledgements

Preface

Chapter 1

Introduction

Chapter 2

The Large Hadron Collider

2.1

A Short Introduction to Lhc

2.2

Impediments to High Luminosity

Chapter 3

Overview of the ATLAS

Detector

3.1

Kinematic Definitions

3.2

ATLAS Magnets and Magnetic Field

3.2.1

Central Solenoid

3.2.2

Toroid

3.3

Tracking System

3.3.1

Overview of the ATLAS Inner Detector

3.3.2

Pixel Detector

_{Prof. Vincenzo Cavasinni}

_{- University of Pisa}

_{Prof. Kenichi Konishi}

_{University of Pisa}

_{Prof. Peter Loch}

_{- University of Arizona}